Recently, an abrasive is employed for a variety of purposes. Above all, highly precise surface polishing is required for glass substrates such as a glass substrate for an optical disk, a glass substrate for a magnetic disk, a glass substrate for a photomask to be employed for semiconductor fabrication, a glass substrate for LCD and the like, an optical lens and the like.
For surface polishing for them, rare earth oxides, especially, a cerium-based abrasive containing cerium oxide as a mainly component (hereinafter referred as to a cerium-based abrasive) has been employed. Because as compared with zirconium oxide and silicon dioxide, cerium oxide is excellent in polishing properties for polishing glass. For example, since it has a high polishing speed as compared with zirconium oxide, silicon dioxide, aluminium oxide, and the like, cerium oxide is capable of performing polishing faster. Further, since cerium oxide has a hardness not so high, smoother glass surface can be obtained after polishing by using cerium oxide for polishing.
However, the polishing properties of the cerium-based abrasive are determined depending on a large number of factors such as contents of cerium oxide, fluorine (F) and the like, a specific surface area and the like. Consequently, to control the polishing properties, there are problems that the respective properties have to be precisely evaluated and comprehensively judged.
Further, the production conditions of the cerium-based abrasive are also important. For example, as one of most important steps, there is a roasting step in the latter half of the production process. The roasting step is a step of heating raw materials for the cerium-based abrasive at a high temperature in an oxidative atmosphere and in the step, fluorine, one of important factors determining the polishing properties, is easy to be dissipated from raw materials. Consequently, the roasting step has to be properly controlled. Especially, in the case of using an apparatus capable of carrying out continuous roasting, it is difficult to quickly and precisely judge the roasting state, thereby leaving difficulty in control as a problem.
The invention is developed to deal with such problems and aims to provide a cerium-based abrasive and a production method of the cerium-based abrasive excellent in polishing properties.